A Bridge to Your First Computer Science Course

A Bridge to Your First Computer Science Course Prof. H.E. Dunsmore while loops do-while loops for loops

What’s Missing? Lots of Data • You know how to make a decision and execute one statement or another • Problems up to now involved working on a fixed set of input or small number of values • Next power up: perform repetitive actions • Two ways to think about repetition: • Perform the same operation on different data • Accumulate information over a set of data

Repetition Concept • Repetition broken into two parts • Body of code that gets repeatedly executed • Condition (boolean) to determine when to stop • How to construct the body so that it does something different/useful each time it is run? • The state of the computation must change with each iteration (otherwise nothing is done)

Two Forms of Iteration • Indefinite: loop until “done”; no advance knowledge of how many iterations will be required • Definite: loop a given number of times; used when the iterations are controlled by a counter or size or limit

Java Repetition Constructs • while loop • Check a boolean condition • If true, execute a block of statements • Repeat • do-while loop • Execute a block of statements • If a boolean condition is true, repeat • for loop

Problem: Odd or Even • Write a program that reads integers from standard input; for each integer print a message indicating “odd” or “even” • Stop reading when no more integers • Scanner method hasNextInt() returns true if there is another integer available, else returns false • Scanner method hasNextInt() returns false at EOF; also returns false if something other than an integer found

Solution: Odd or Even import java.util.Scanner; public class OddOrEven { public static void main(String[] args) { Scanner in = new Scanner(System.in); int number; while (in.hasNextInt()) { number = in.nextInt(); if (number % 2 == 0) System.out.printf("%d is even\n", number); else System.out.printf("%d is odd\n", number); } } }

Problem: Summer • Read a sequence of integers from the standard input and compute their sum • Two problems: • How do we know when we’re done? • How do we accumulate the sum? • Also count the number of values read

Solution: Summer import java.util.Scanner; public class Summer { public static void main(String[] args) { Scanner in = new Scanner(System.in); int number; //number that is input int sum = 0; // sum of values int c = 0; // how many values read while (in.hasNextInt()) { number = in.nextInt(); c = c + 1; sum = sum + number; } System.out.printf("sum of %d values is %d\n", c, sum); } }

The while Loop while (boolean-expression) { statements; } • Test boolean-expression first • If true, do statements in body • Repeat from 1.

Problem: Palindrome • Write a method in class Palindrome booleanisPalindrome(String s) • to test if s is a palindrome, a string that reads the same backwards as forwards • Approach 1: Use a while loop

Strategy: Palindrome • Compare first and last characters; differ? False • Strip off first and last characters • Repeat until length < 2; return true • Test input: • “level” (true) • “racecar” (true) • “henway” (false) • “x”, “aba”, “abba” (all true) • “” (empty string (true)) • null (null value (true))

Solution: Palindrome import java.util.Scanner; public class Palindrome { booleanisPalindrome(String s) { if (s == null || s.length() <= 1) return true; while (s.length() > 1) { char first = s.charAt(0); char last = s.charAt(s.length() - 1); if (first != last) return false; s = s.substring(1, s.length() - 1); } return true; } }

Solution: PalindromeTest import junit.framework.TestCase; public class PalindromeTest extends TestCase { public void testIsPalindrome() { Palindrome p = new Palindrome(); assertEquals(true, p.isPalindrome("")); assertEquals(true, p.isPalindrome(null)); assertEquals(true, p.isPalindrome("x")); assertEquals(true, p.isPalindrome("xx")); assertEquals(false, p.isPalindrome("xy")); assertEquals(true, p.isPalindrome("level")); assertEquals(false, p.isPalindrome("henway")); assertEquals(true, p.isPalindrome("racecar")); } }

Problem: Reverse • Add a method String reverse(String s) • to Palindrome to reverse a String • isPalindrome could simply be… booleanisPalindrome(String s) { return s.equals(reverse(s)); }

Pro Tip: Compound Assignment • Common assignment statements: x = x + y; a = a – b; s = s + "\n"; // s is a string • Java provides shortcut to save keystrokes: x += y; a -= b; s += "\n"; • Available for all (or most) binary operators

Pro Tip 2: Increment/Decrement Operators • A refinement for an even more common case: x = x + 1; a = a – 1; • Java provides even more keystroke savings: x++; a--; • Also: ++x; --a;

Post- and Pre- Increment/Decrement • x++ increments x by one, but the expression value is the original x int x = 0; System.out.println(x++); System.out.println(x); • Prints 0, then 1 • ++x increments x by one, and its value is the new x intx = 0; System.out.println(++x); System.out.println(x); • Prints 1, then 1 • x++ and x-- are very common idioms (cf. C++) • Life becomes messy if an expression contains multiple pre- and post- increment and decrement operators

Problem: WhileDefinite • Problem: Print “hello” 10 times using a while loop • Illustrates using a while loop to implement a definite iteration

Solution: WhileDefinite public class WhileDefinite { public static void main(String[] args) { int n = 0; while (n < 10) { System.out.printf("hello (#%d)\n", n); n++; } } }

The Loop Parts e1. Initialize public class WhileDefinite { public static void main(String[] args) { int n = 0; while (n < 10) { System.out.printf("hello (#%d)\n", n); n++; } } } for (int n = 0; n < 10; n++) System.out.printf("hello (#%d)\n", n); e2. Test e3. Update

Definite Iteration: for loop • Very general form: for (e1; e2; e3) { statements; } • Sequence of actions: • Evaluate expression e1 (once only). • Evaluate e2. If true, execute statement body. • Evaluate e3. • Return to step 2.

Common Practices • To loop n times, go from 0 to n-1 for (inti = 0; i < n; i++) { statements; } • Works well for strings (and arrays): 0-based • To print the characters in a String s: String s = "hello there world"; for (inti = 0; i < s.length(); i++) System.out.printf( "s.charAt(%d) = '%c'\n", i, s.charAt(i));

The do-while Loop do { statements; } while (boolean-expression) • Execute statements in body • Test boolean-expression • If true, repeat from 1

Problem: Prompting the User • Write a program, Prompter, that prompts the user for an even number • Continue prompting until an even number is provided • Show alternate implementation using standard while loop with sentinel

Solution 1: Prompter1 import java.util.Scanner; public class Prompter1 { public static void main(String[] args) { Scanner in = new Scanner(System.in); // Prompt for an even number using do-while... int n; do { System.out.printf("Please enter an even number: "); n = in.nextInt(); } while (n % 2 == 1); System.out.printf("Thank you for entering the even number %d\n", n); } }

Solution 2: Prompter2 import java.util.Scanner; public class Prompter2 { public static void main(String[] args) { Scanner in = new Scanner(System.in); int n = 0; // Prompt for an even number with while, using sentinel... boolean done = false; while (!done) { System.out.printf("Please enter an even number: "); n = in.nextInt(); if (n % 2 == 0) done = true; else System.out.printf("The number %d is not even.\n", n); } System.out.printf("Thank you for entering the even number %d\n", n); } }

Problem: Palindrome (Redone) • Write a method in class Palindrome booleanisPalindrome(String s) • to test if s is a palindrome (reads the same backwards as forward) • Approach 2: Use a for loop

Palindrome: Strategy • Iterate through the first half of the string • Compare current character to corresponding character at other end of string • Consider table of indexes i and j for “racecar” • Find pattern, generalize to solution

Palindrome: Solution booleanisPalindrome(String s) { if (s == null) return true; for (inti = 0; i < s.length() / 2; i++) if (s.charAt(i) != s.charAt(s.length() - 1 - i)) return false; return true; }

Infinite Loop public class InfiniteLoop { public static void main(String[] args) { int n = 1; while (n < 100) { System.out.printf("n = %d\n", n); // forgot to increment n } } }

Fencepost Error public class FencePostError { public static void main(String[] args) { for (inti = 0; i <= 5; i++) System.out.printf( "print this line 5 times (%d)\n", i); } }

Skipped Loop import java.util.Scanner; public class SkippedLoop { public static void main(String[] args) { Scanner in = new Scanner(System.in); int number = 0; int sum = 0; // read ints from user until zero, then print sum while (number > 0) { sum += number; number = in.nextInt(); } System.out.printf("sum = %d\n", sum); } }

Misplaced Semicolon public class MisplacedSemicolon { public static void main(String[] args) { inti = 10; while (--i >= 0); { System.out.printf("message #%d\n", i); } } }

Nested Loops • Just like you can nest if statements • You can also nest loops • Inner loop is run completely for each iteration of outer loop: public class Nested { public static void main(String[] args) { for (inti = 0; i < 5; i++) for (int j = 0; j < 5; j++) System.out.printf("i = %d, j = %d\n",i, j); } }

Problem: Draw Divisor Pattern • Print an NxN table • The entry at row i and column j has an * if i divides j or j divides I • Example for N == 5 1 2 3 4 5 1 * * * * * 2 * * * 3 * * 4 * * * 5 * *

Solution: Draw Divisor Pattern public class DivisorPattern { public static void main(String[] args) { int N = Integer.parseInt(args[0]); System.out.printf(" "); for (inti = 1; i <= N; i++) System.out.printf("%3d", i); System.out.printf("\n"); for (inti = 1; i <= N; i++) { System.out.printf("%3d", i); for (int j = 1; j <= N; j++) { if (i % j == 0 || j % i == 0) System.out.printf(" *"); else System.out.printf(" "); } System.out.printf("\n"); } } }

A Bridge to Your First Computer Science Course